Fillet Gauge
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Fillet weld symbol: what it means on a drawing

Fillet Gauge editorial teamSources checked 4 min read

The fillet weld symbol is a small right triangle hung on the reference line of a welding symbol. It calls for a fillet weld, the triangular weld that joins two surfaces meeting at about a right angle in a T, lap or corner joint. Below the line it means the arrow side; above the line, the other side.

Two T-joints drawn end on. A: a fillet triangle below the reference line marked 1/4 and a fillet on the arrow side only. B: triangles above and below the line and fillets on both sides of the stem
The triangle's side of the line picks the side of the joint. Numbers are invented.
Use this diagram

How to read a fillet weld symbol

  1. Find the side. A triangle below the reference line puts the fillet on the side of the joint the arrow touches. Above the line puts it on the far side. One triangle on each side of the line means a fillet on both sides of that one joint.
  2. Read the size on the left. The number to the left of the triangle is the fillet size, the leg length. 1/4 means a fillet with 1/4 legs. An unequal-leg fillet shows both legs, such as 1/4 x 3/8, and the drawing shows which leg goes on which member.
  3. Read the right side. No number means the weld runs the full length of the joint the arrow points to. A single number is the weld length. Two numbers joined by a hyphen, such as 2-6, are length and pitch: a stitch weld.
  4. Check the bend and the tail. A circle where the arrow meets the line means weld all around. A flag there means a field weld. A line beside the triangle is a contour mark for the face shape. The tail carries the process or a WPS reference when one is given.
  5. Check the units. The symbol doesn't set them. The title block does.

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Symbols - Welding and NDE

On a welding symbol, what does a circle at the junction of the arrow and the reference line indicate?

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Common mix-ups

  • The upright leg. The triangle's vertical leg is always drawn on the left, whichever plate is welded. It tells you nothing about the joint. The arrow and the side of the line do.
  • Leg or throat. The number is the leg size, not the throat. For an equal-leg fillet with a flat face, the theoretical throat is about 0.707 times the leg, so a 1/4 fillet has a throat of about 0.177. The fillet weld calculator works it out for equal and unequal legs.
  • Fillet or bevel. A fillet triangle sits with its flat top edge on the reference line. A bevel groove mark touches the line at one point and opens away from it. Read the shape before the numbers.
  • Both sides or two joints. Triangles above and below the line mean both sides of the same joint. Another joint on the same part needs its own arrow.

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Fillet symbols on the CWI exam

Symbols, Welding and NDE is at least 10% of CWI Part A, and Part A is closed book, so you read fillet sizes, sides and lengths from memory. Drawings come back in Part B, where Utilization of Specifications and Drawings is at least 8% of the practical exam. The fillet is the symbol you will see most on structural shop drawings, so it pays to read it without thinking.

Practice it

Build a few fillet symbols in the welding symbol builder: pick the side, size, length and pitch and it reads the symbol back in plain words. Then see the fillet next to every other mark on the welding symbols chart, and work through all-around, field and stitch notation together in fillet weld symbols explained.

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Take the free 10-question check. It's weighted to the Part A outline, scored by domain and needs no signup, so you see whether symbols or another domain costs you the most points. The Part A course gives you a lesson on reading welding and NDE symbols and on every other exam subject, two full-length timed practice exams, and a worked explanation with every answer.

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Facts checked 24 October 2026 against the sources listed above. If something has changed, tell us and we will correct it within 2 business days.

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